Due to upgrading of portable electronic devices, such as a notebook computer, a cell phone, and a PDA, the power consumption of such devices has been increasing in the recent years. Currently the major power source of such portable electronic devices is a lithium-ion secondary battery; however its energy density cannot keep up with the recent power consumption increase, and therefore is an obstacle to further upgrading of portable electronic devices.
As a next-generation power source with high energy density replacing a lithium-ion secondary battery, a polymer electrolyte fuel cell has drawn attention. A polymer electrolyte fuel cell is constituted by stacking a number of single cells. FIG. 1 shows a typical structure of a single cell. In FIG. 1 a polyelectrolyte membrane (ion exchange membrane) 10 is sandwiched from both sides by an anode catalyst layer 20 and a cathode catalyst layer 21, further the catalyst layers 20, 21 are sandwiched from both sides by an anode gas diffusion layer 40 and a cathode gas diffusion layer 41 (the gas diffusion layer being also called as a “porous substrate”, or as a “carbon fiber-made current collector”), and the outer surfaces of the gas diffusion layers 40, 41 are open to gas channels (a fuel gas channel 50, and an oxygen-containing gas channel 51) constituted by separators 60, 61. A fuel gas (H2, etc.) introduced through a channel 50 passes the anode gas diffusion layer 40 to reach the anode catalyst layer 20, where the fuel gas emits an electron to produce a proton (H+) according to the following anode reaction. The proton passes the polyelectrolyte membrane 10 to reach the cathode catalyst layer 21, where the proton receives an electron according to the following cathode reaction to produce H2O. The following are an anode reaction and a cathode reaction in the case that a fuel gas is hydrogen:anode reaction: H2→2H++2e−cathode reaction: ½O2+2H++2e−→H2O
As a fuel there are a hydrogen containing substance, such as hydrogen and sodium borohydride, an alcohol, such as methanol and ethanol, and other organic substance fuels. Among others, methanol has high volumetric energy density, is liquid and easy to carry, and therefore is suitable for use in a small sized portable device. If methanol is used as a fuel, usually methanol and water are reacted at an anode, and an aqueous methanol solution is supplied to an anode.
In an actual fuel cell, in addition to the above main reaction, side reactions take place. Typically hydrogen peroxide (H2O2) is produced. Although the production mechanism is not fully understood, it may be as follows: the production of hydrogen peroxide can take place both at an anode and at a cathode, and, for example, at a cathode hydrogen peroxide is seemingly produced by an incomplete reducing reaction of oxygen according to the following formula:side reaction at cathode: O2+2H++2e−→2H2O2 
While, it is believed that at the anode oxygen contained as an impurity or added intentionally in a gas, or dissolved in an electrolyte at a cathode and diffused to the anode should participate in a reaction, which reaction formula is identical with the aforedescribed side reaction at the cathode or as represented by the following formula:side reaction at anode: 2M−H+O2−→2M+H2O2 
Wherein M represents a catalytic metal used in the anode, and M−H represents a state in which hydrogen is adsorbed on the catalytic metal. Usually as a catalytic metal a noble metal such as platinum (Pt) is utilized.
The hydrogen peroxide generated at the electrodes is liberated from the electrodes by diffusion or otherwise and migrates into an electrolyte. The hydrogen peroxide is a strongly oxidizing substance and oxidizes various organic substances constituting the electrolyte. No detailed mechanism thereof has been clarified, it is believed however that, in most cases, hydrogen peroxide is activated to a radical, and the generated hydrogen peroxide radical acts as a primary reactive substance of an oxidation reaction. Namely, a radical generated by a reaction as described below presumably withdraws a hydrogen from an organic substance of the electrolyte, or breaks any other bond. Although a cause for activation to a radical is not exactly clear, it has been considered that it is catalyzed by contact with a heavy metal ion. Further, it is also believed that a radical can be formed by heat, light, etc.H2O2→2.OH or H2O2→.H+.OOH
Several countermeasures for preventing deterioration of a polyelectrolyte membrane by a peroxide generated in an electrode layer have been proposed.
Patent Literature 1 proposes, in order to prevent deterioration of a polyelectrolyte membrane by a peroxide generated in an electrode layer, a solid polyelectrolyte, wherein a transition metal oxide having a catalytic activity for decomposing catalytically a peroxide, especially manganese oxide, ruthenium oxide, cobalt oxide, nickel oxide, chromium oxide, iridium oxide or lead oxide, is distributed in a polyelectrolyte membrane.
Patent Literature 2 proposes, in order to enhance the resistance to a hydrogen peroxide or peroxide radicals of a polyelectrolyte membrane containing a sulfonic acid group in a polymer electrolyte fuel cell, an electrolyte membrane for a polymer electrolyte fuel cell, wherein fine particles of a poorly-soluble cerium compound are admixed in the polyelectrolyte membrane.
Patent Literature 3 proposes, in order to improve the durability against a hydrogen peroxide or peroxide radicals and to enhance the mechanical strength of an electrolyte membrane, an electrolyte membrane for a polymer electrolyte fuel cell, wherein the polyelectrolyte membrane containing a cerium ion or a manganese ion is reinforced by a porous membrane or the like.